On Thu, Apr 26, 2018 at 11:34:19AM +0100, Will Deacon wrote:
> @@ -290,58 +312,50 @@ void queued_spin_lock_slowpath(struct qspinlock *lock,
> u32 val)
> }
>
> /*
> + * If we observe any contention; queue.
> + */
> + if (val & ~_Q_LOCKED_MASK)
> + goto queue;
> +
> + /*
> * trylock || pending
> *
> * 0,0,0 -> 0,0,1 ; trylock
> * 0,0,1 -> 0,1,1 ; pending
> */
> + val = atomic_fetch_or_acquire(_Q_PENDING_VAL, &lock->val);
> + if (!(val & ~_Q_LOCKED_MASK)) {
> /*
> + * we're pending, wait for the owner to go away.
> + *
> + * *,1,1 -> *,1,0
Tail must be 0 here, right?
> + *
> + * this wait loop must be a load-acquire such that we match the
> + * store-release that clears the locked bit and create lock
> + * sequentiality; this is because not all
> + * clear_pending_set_locked() implementations imply full
> + * barriers.
> */
> + if (val & _Q_LOCKED_MASK) {
> + smp_cond_load_acquire(&lock->val.counter,
> + !(VAL & _Q_LOCKED_MASK));
> + }
>
> /*
> + * take ownership and clear the pending bit.
> + *
> + * *,1,0 -> *,0,1
> */
Idem.
> + clear_pending_set_locked(lock);
> return;
> + }
>
> /*
> + * If pending was clear but there are waiters in the queue, then
> + * we need to undo our setting of pending before we queue ourselves.
> */
> + if (!(val & _Q_PENDING_MASK))
> + clear_pending(lock);
This is the branch for when we have !0 tail.
>
> /*
> * End of pending bit optimistic spinning and beginning of MCS
> @@ -445,15 +459,15 @@ void queued_spin_lock_slowpath(struct qspinlock *lock,
> u32 val)
> * claim the lock:
> *
> * n,0,0 -> 0,0,1 : lock, uncontended
> + * *,*,0 -> *,*,1 : lock, contended
> *
> + * If the queue head is the only one in the queue (lock value == tail)
> + * and nobody is pending, clear the tail code and grab the lock.
> + * Otherwise, we only need to grab the lock.
> */
> for (;;) {
> /* In the PV case we might already have _Q_LOCKED_VAL set */
> + if ((val & _Q_TAIL_MASK) != tail || (val & _Q_PENDING_MASK)) {
> set_locked(lock);
> break;
> }
This one hunk is terrible on the brain. I'm fairly sure I get it, but I
feel that comment can use help. Or at least, I need help reading it.
I'll try and cook up something when my brain starts working again.
